Cuprous oxide powders have been used in various fields as the raw materials of copper powders for electronic materials, preservatives for ship bottom paints (antifouling paints), bactericides, agricultural chemicals, conductive coating materials, copper plating solutions, coloring materials for ceramics, catalysts, rectifiers, solar cells and so forth.
As examples of cuprous oxide powders used as the raw materials of copper powders for electronic materials, cuprous oxide powders are used as the raw materials of copper powders which are used for copper pastes for forming internal electrodes of ceramic multilayer electronic parts, such as ceramic multilayer capacitors and ceramic multilayer inductors, and external electrodes of ceramic multilayer capacitors and ceramic multilayer inductors.
In recent years, it is desired to reduce the thickness of electrodes of ceramic multilayer capacitors with the higher capacity and smaller size thereof. For that reason, it is desired to provide a copper powder of fine monodisperse copper particles having small particle diameters and a narrow particle size distribution (a small variation in particle diameters) as a metallic material for electrodes of ceramic multilayer capacitors and so forth.
Typical copper powders for electronic materials are produced by dry processes, such as atomizing processes, or wet processes, such as chemical reduction processes. In atomizing processes, it is possible to decrease impurities in copper powders by increasing the purity of the raw materials of the copper powders. However, since there has not been established techniques for efficiently obtaining copper powders of fine copper particles having small particle diameters and a narrow particles size distribution (a small variation in particle diameters), it is difficult to prevent coarse grains from being mixed therein, so that it is required to remove the coarse grains by sifting or the like. On the other hand, chemical reduction processes are suitable for the production of copper powders of fine monodisperse copper particles having small particle diameters and a narrow particle size distribution (a small variation in particle diameters).
As methods for producing fine copper powders by chemical reduction processes, there are proposed a method for producing a fine copper powder by reducing cuprous oxide (which is obtained by reducing copper hydroxide, which is deposited from an aqueous copper salt solution, by a reducing sugar being a week reducing agent) to metallic copper by a reducing agent of hydrazine (see, e.g., Japanese Patent Laid-Open No. 4-116109), a method for mixing a pH-controlled fluid dispersion of copper oxide with a reducing agent solution having a predetermined concentration to deposit fine particles of metallic copper by direct reduction (see, e.g., Japanese Patent Laid-Open No. 2006-22394), and a method for adding an alkali solution and a reducing agent solution (containing no carbon and no chlorine) to an aqueous solution containing cupric ions to deposit cuprous oxide particles by reduction to obtain a copper powder by reducing the deposited cuprous oxide particles (see, e.g., Japanese Patent Laid-Open No. 2010-59001).
However, cuprous oxide particles obtained by the method proposed in Japanese Patent Laid-Open No. 4-116109 have large particle diameters of about 4 μm, so that it is not possible to obtain a cuprous oxide powder having particle diameters of sub-microns. In the method proposed in Japanese Patent Laid-Open No. 2006-22394, fine metallic copper particles are deposited by directly reducing copper oxide, so that it is not possible to obtain cuprous oxide powder having particle diameters of sub-microns. In the method proposed in Japanese Patent Laid-Open No. 2010-59001, although it is possible to obtain a cuprous oxide powder having particle diameters of sub-microns and having a narrow particle size distribution (a small variation in particle diameters), it is desired to obtain a cuprous oxide powder having a smaller particle diameter.